This invention relates to a projection type display apparatus for enlarging an image displayed on a display screen of a display device and projecting the enlarged image on a projection screen or a projection board, and a projection optical system thereof, or in particular to a projection type display apparatus for projecting obliquely on the projection screen an image displayed on the display screen of a display device, an optical system thereof, and a method of machining a molded plastic lens constituting the optical system.
In the projection type display apparatus for displaying in an enlarged form, on a projection screen or a projection board, an image on a display screen of a liquid crystal panel of reflection type or transmission type or a displayed image of a display device with an array of a plurality of micro mirrors arranged thereon, it is important not only to produce a sufficiently enlarged image on the projection screen but also to prevent the shadow of a presenter from entering the projection screen and the enlarged image light from intruding directly into the eyes of the presenter. To meet this requirement, a projection optical system of what is called the short projection type in which the distance between the projection type display apparatus and the projection screen is decreased has begun to be placed on the market. This projection optical system is so configured that the enlarged image light enters the projection screen from an oblique direction (for example, JP-A-2008-250296).
Means for optically adjusting the oblique projection optical system using a curved mirror for oblique projection is also known (for example, JP-A-2002-35077).
On the other hand, a projection type display apparatus of rear projection type is known in which a mirror for returning optical path is interposed between the projection type display apparatus and the projection screen to decrease the apparent projection distance (for example, JP-A-2006-259252).